Madonna, who only recently joined Instagram, appears to have broken the Facebook-owned photo-sharing site's rules.

The mononymous pop celebrity posted a warning addressed to her that accused her of violating Instagram's community guidelines.

Instagram's rules ban nudity and the posting of images that don't belong to the user posting them, among other restrictions.

Madonna has more than 150,000 followers on Instagram. Many of them leapt to the assumption that Madonna was being punished for photos that displayed cleavage or ample portions of the star's buttocks. It's possible, however, that Instagram took action for other reasons.

For example, Madonna posted at least two photos of Frida Kahlo, the Mexican artist whom she has long cited as an inspiration.

Of course, it's difficult to know if those were actual copyright violations. Madonna may in fact own the photos in question; she's a known collector of Kahlo's works and other Kahlo-related material.

Facebook did not immediately respond to a request for comment. The company typically does not comment on individual users' use of its services.

Instagram drew criticism late last year for revising its terms of service in a way that suggested it might sell users' photos. CEO Kevin Systrom apologized and the service retracted some of the more controversial terms.